A method of this kind is known from SMRM Abstracts (San Fransisco, 212 (1988)) and from Chem. Phys. Lett., 69 (1980), pp. 567-570. According to the known method, a magnetic gradient field is switched on and off before and after the last RF pulse. The time integral over this field after the last pulse amounts to exactly twice the corresponding integral before this pulse. It is thus achieved that only double quantum coherences influence the spin resonance signal. Single quantum coherences, notably the signals originating from water-bound protons, are then strongly suppressed.
In the cited SMRM publication this method is used to determine the concentration of lactic acid or lactate independently from the concentration of fat or lipids. The Larmor frequencies of protons bound to fat and to lactic acid are very close to one another. In order to enable separation of these two components, according to the known method a two-dimensional frequency spectrum is generated. To this end, the so-called evolution time between the second and the third RF pulse is varied in steps, the spin resonance signals thus acquired being used to derive therefrom a two-dimensional frequency spectrum by way of two-dimensional Fourier transformation, fat and lactic acid being suitably separable in the spectrum.
Two-dimensional frequency spectra require very long overall measuring periods and are susceptible to motions in the examination zone. Therefore, it would be worthwhile to pursue determination of the lactic acid concentration independently from the fat concentration in a one-dimensional frequency spectrum. This is hampered by the fact that one frequency component of protons bound to lactic acid is situated in the immediate vicinity of the Larmor frequency of water-bound protons, and that the other frequency component of lactic acid directly neighbors a fat component which is also contained in the double quantum spectrum.